Fix failure to guarantee that a checkpoint will write out pg_clog updates

for transaction commits that occurred just before the checkpoint.  This is
an EXTREMELY serious bug --- kudos to Satoshi Okada for creating a
reproducible test case to prove its existence.

diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index 4794c761c3a56cb53ce55d05e22f3872487b9e9d..594a2fcca1ac7db9c8a136f26c5563383245b146 100644 (file)
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *       $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.177 2004/08/03 15:57:26 tgl Exp $
+ *       $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.178 2004/08/11 04:07:15 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -574,13 +574,28 @@ RecordTransactionCommit(void)
                START_CRIT_SECTION();
 
                /*
-                * We only need to log the commit in XLOG if the transaction made
-                * any transaction-controlled XLOG entries or will delete files.
+                * If our transaction made any transaction-controlled XLOG entries,
+                * we need to lock out checkpoint start between writing our XLOG
+                * record and updating pg_clog.  Otherwise it is possible for the
+                * checkpoint to set REDO after the XLOG record but fail to flush the
+                * pg_clog update to disk, leading to loss of the transaction commit
+                * if we crash a little later.  Slightly klugy fix for problem
+                * discovered 2004-08-10.
+                *
                 * (If it made no transaction-controlled XLOG entries, its XID
                 * appears nowhere in permanent storage, so no one else will ever care
-                * if it committed.)
+                * if it committed; so it doesn't matter if we lose the commit flag.)
+                *
+                * Note we only need a shared lock.
                 */
                madeTCentries = (MyLastRecPtr.xrecoff != 0);
+               if (madeTCentries)
+                       LWLockAcquire(CheckpointStartLock, LW_SHARED);
+
+               /*
+                * We only need to log the commit in XLOG if the transaction made
+                * any transaction-controlled XLOG entries or will delete files.
+                */
                if (madeTCentries || nrels > 0)
                {
                        XLogRecData rdata[3];
@@ -668,6 +683,10 @@ RecordTransactionCommit(void)
                        TransactionIdCommitTree(nchildren, children);
                }
 
+               /* Unlock checkpoint lock if we acquired it */
+               if (madeTCentries)
+                       LWLockRelease(CheckpointStartLock);
+
                END_CRIT_SECTION();
        }
 
@@ -850,6 +869,8 @@ RecordTransactionAbort(void)
                 *
                 * We do not flush XLOG to disk unless deleting files, since the
                 * default assumption after a crash would be that we aborted, anyway.
+                * For the same reason, we don't need to worry about interlocking
+                * against checkpoint start.
                 */
                if (MyLastRecPtr.xrecoff != 0 || nrels > 0)
                {

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 28fb4c733a6032613428ac8b83a8c528e83b4cb5..32ade5d7590f31b66716542d59888337c8ca7709 100644 (file)
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.158 2004/08/09 16:26:01 tgl Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.159 2004/08/11 04:07:15 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -4699,6 +4699,15 @@ CreateCheckPoint(bool shutdown, bool force)
        checkPoint.ThisTimeLineID = ThisTimeLineID;
        checkPoint.time = time(NULL);
 
+       /*
+        * We must hold CheckpointStartLock while determining the checkpoint
+        * REDO pointer.  This ensures that any concurrent transaction commits
+        * will be either not yet logged, or logged and recorded in pg_clog.
+        * See notes in RecordTransactionCommit().
+        */
+       LWLockAcquire(CheckpointStartLock, LW_EXCLUSIVE);
+
+       /* And we need WALInsertLock too */
        LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
 
        /*
@@ -4731,6 +4740,7 @@ CreateCheckPoint(bool shutdown, bool force)
                        ControlFile->checkPointCopy.redo.xrecoff)
                {
                        LWLockRelease(WALInsertLock);
+                       LWLockRelease(CheckpointStartLock);
                        LWLockRelease(CheckpointLock);
                        END_CRIT_SECTION();
                        return;
@@ -4789,6 +4799,9 @@ CreateCheckPoint(bool shutdown, bool force)
         * GetSnapshotData needs to get XidGenLock while holding SInvalLock,
         * so there's a risk of deadlock. Need to find a better solution.  See
         * pgsql-hackers discussion of 17-Dec-01.
+        *
+        * XXX actually, the whole UNDO code is dead code and unlikely to ever
+        * be revived, so the lack of a good solution here is not troubling.
         */
 #ifdef NOT_USED
        checkPoint.undo = GetUndoRecPtr();
@@ -4798,11 +4811,13 @@ CreateCheckPoint(bool shutdown, bool force)
 #endif
 
        /*
-        * Now we can release insert lock, allowing other xacts to proceed
-        * even while we are flushing disk buffers.
+        * Now we can release insert lock and checkpoint start lock, allowing
+        * other xacts to proceed even while we are flushing disk buffers.
         */
        LWLockRelease(WALInsertLock);
 
+       LWLockRelease(CheckpointStartLock);
+
        /*
         * Get the other info we need for the checkpoint record.
         */

diff --git a/src/include/storage/lwlock.h b/src/include/storage/lwlock.h
index b1f6fc9510349755f3b51709bbd25104c1af5363..7b08231e5103e9faa56a04d388f143f93478a560 100644 (file)
--- a/src/include/storage/lwlock.h
+++ b/src/include/storage/lwlock.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2003, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/storage/lwlock.h,v 1.12 2004/06/11 16:43:24 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/storage/lwlock.h,v 1.13 2004/08/11 04:07:16 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -36,6 +36,7 @@ typedef enum LWLockId
        WALWriteLock,
        ControlFileLock,
        CheckpointLock,
+       CheckpointStartLock,
        RelCacheInitLock,
        BgWriterCommLock,